cmd/objdump: add arm disassembler

Fixes #7452.

LGTM=minux, iant
R=minux, iant
CC=golang-codereviews
https://golang.org/cl/104770046

src/cmd/objdump/Makefile

+all: x86.go armasm.go
+
 x86.go: bundle
 	./bundle -p main -x x86_ rsc.io/x86/x86asm | gofmt >x86.go
 
+armasm.go: bundle
+	./bundle -p main -x arm_ rsc.io/arm/armasm | gofmt >armasm.go
+
 bundle:
 	go build -o bundle code.google.com/p/rsc/cmd/bundle

src/cmd/objdump/main.go

@@ -42,6 +42,7 @@ import (
 	"debug/macho"
 	"debug/pe"
 	"debug/plan9obj"
+	"encoding/binary"
 	"flag"
 	"fmt"
 	"io"
@@ -135,7 +136,7 @@ func main() {
 
 	if flag.NArg() == 1 {
 		// disassembly of entire object - our format
-		dump(tab, lookup, disasm, syms, textData, textStart)
+		dump(tab, lookup, disasm, goarch, syms, textData, textStart)
 		os.Exit(exitCode)
 	}
 
@@ -152,7 +153,7 @@ func base(path string) string {
 	return path
 }
 
-func dump(tab *gosym.Table, lookup lookupFunc, disasm disasmFunc, syms []Sym, textData []byte, textStart uint64) {
+func dump(tab *gosym.Table, lookup lookupFunc, disasm disasmFunc, goarch string, syms []Sym, textData []byte, textStart uint64) {
 	stdout := bufio.NewWriter(os.Stdout)
 	defer stdout.Flush()
 
@@ -178,7 +179,14 @@ func dump(tab *gosym.Table, lookup lookupFunc, disasm disasmFunc, syms []Sym, te
 			i := pc - textStart
 			text, size := disasm(textData[i:end-textStart], pc, lookup)
 			file, line, _ := tab.PCToLine(pc)
-			fmt.Fprintf(tw, "\t%s:%d\t%#x\t%x\t%s\n", base(file), line, pc, textData[i:i+uint64(size)], text)
+
+			// ARM is word-based, so show actual word hex, not byte hex.
+			// Since ARM is little endian, they're different.
+			if goarch == "arm" && size == 4 {
+				fmt.Fprintf(tw, "\t%s:%d\t%#x\t%08x\t%s\n", base(file), line, pc, binary.LittleEndian.Uint32(textData[i:i+uint64(size)]), text)
+			} else {
+				fmt.Fprintf(tw, "\t%s:%d\t%#x\t%x\t%s\n", base(file), line, pc, textData[i:i+uint64(size)], text)
+			}
 			pc += uint64(size)
 		}
 		tw.Flush()
@@ -206,19 +214,37 @@ func disasm_x86(code []byte, pc uint64, lookup lookupFunc, arch int) (string, in
 	return text, size
 }
 
+type textReader struct {
+	code []byte
+	pc   uint64
+}
+
+func (r textReader) ReadAt(data []byte, off int64) (n int, err error) {
+	if off < 0 || uint64(off) < r.pc {
+		return 0, io.EOF
+	}
+	d := uint64(off) - r.pc
+	if d >= uint64(len(r.code)) {
+		return 0, io.EOF
+	}
+	n = copy(data, r.code[d:])
+	if n < len(data) {
+		err = io.ErrUnexpectedEOF
+	}
+	return
+}
+
 func disasm_arm(code []byte, pc uint64, lookup lookupFunc) (string, int) {
-	/*
-		inst, size, err := arm_Decode(code, 64)
-		var text string
-		if err != nil || size == 0 || inst.Op == 0 {
-			size = 1
-			text = "?"
-		} else {
-			text = arm_plan9Syntax(inst, pc, lookup)
-		}
-		return text, size
-	*/
-	return "?", 4
+	inst, err := arm_Decode(code, arm_ModeARM)
+	var text string
+	size := inst.Len
+	if err != nil || size == 0 || inst.Op == 0 {
+		size = 4
+		text = "?"
+	} else {
+		text = arm_plan9Syntax(inst, pc, lookup, textReader{code, pc})
+	}
+	return text, size
 }
 
 var disasms = map[string]disasmFunc{

src/cmd/objdump/objdump_test.go

@@ -140,7 +140,7 @@ var x86Need = []string{
 var armNeed = []string{
 	"fmthello.go:6",
 	"TEXT main.main(SB)",
-	"B main.main(SB)",
+	"B.LS main.main(SB)",
 	"BL fmt.Println(SB)",
 	"RET",
 }
@@ -172,7 +172,6 @@ func TestDisasm(t *testing.T) {
 		need = append(need, x86Need...)
 	case "arm":
 		need = append(need, armNeed...)
-		t.Skip("disassembler not ready on arm yet")
 	}
 
 	out, err = exec.Command(exe, "-s", "main.main", hello).CombinedOutput()